In order to cope with a recent increase in communication traffic, a further increase in communication (transmission) capacity has been required. However, in optical communication systems using a single-mode fiber (SMF) which is used in optical communication of related art, a limitation on an increase in capacity is expected. As a technique for exceeding the limitation, research and development of space division multiplexing (SDM) have been actively performed.
As optical fibers for realizing SDM, there are a few-mode fiber (FMF) having the achievement of an increase in capacity by propagating a plurality of modes into one core and loading a signal into each mode, and a multicore fiber (MCF) having the achievement of an increase in capacity by loading a signal into each of a plurality of cores (see Patent Documents 1 to 6 regarding the MCF).
In addition, an example of the MCF generally includes two types of uncoupled MCF in which respective cores transmit information independently of each other, and coupled multicore fiber (C-MCF) in which supermodes are formed by (modes of) the respective cores being coupled to each other, and information is transmitted to each supermode (see Non-Patent Documents 1 to 3 regarding the C-MCF). The C-MCF is one of fibers for mode division multiplexing (MDM) transmission.
Particularly, in a few-mode transmission, it is important to reduce differential mode group delay (DGD) in order to reduce a load of signal processing such as multiple-input and multiple-output (MIMO). In the C-MCF, an examination aiming at reducing the DGD by enhancing coupling between the supermodes has been performed (see Non-Patent Documents 2 and 3).
Particularly, in the C-MCF, it has been shown experimentally that the DGD in MIMO reception can be reduced by using coupling between modes due to random perturbation in a fiber. Therefore, the C-MCF has attracted considerable attention (see Non-Patent Document 3). In addition, Non-Patent Document 4 discloses an example in which, as in a report produced by the group of the inventors, holes are added around the center of a core in an identical type of 3-core C-MCF.